Microwave material characterization of alkali-silica reaction (ASR) gel in cementitious materials

Since alkali-silica reaction (ASR) was recognized as a durability challenge in cement-based materials over 70 years ago, numerous methods have been utilized to prevent, detect, and mitigate this issue. However, quantifying the amount of produced ASR byproducts (i.e., ASR gel) in-service is still of great interest in the infrastructure industry. The overarching objective of this dissertation is to bring a new understanding to the fundamentals of ASR formation from a microwave dielectric property characterization point-of-view, and more importantly, to investigate the potential for devising a microwave nondestructive testing approach for ASR gel detection and evaluation. To this end, a comprehensive dielectric mixing model was developed with the potential for predicting the effective dielectric constant of mortar samples with and without the presence of ASR gel. To provide pertinent inputs to the model, critical factors on the influence of ASR gel formation on dielectric and reflection properties of several mortar samples were investigated at R, S, and X-band. Effects of humidity, alkali content, and long-term curing conditions on ASR-prone mortars were also investigated. Additionally, dielectric properties of chemically different synthetic ASR gel were also determined. All of these, collectively, served as critical inputs to the mixing model. The resulting developed dielectric mixing model has the potential to be further utilized to quantify the amount of produced ASR gel in cement-based materials. This methodology, once becomes more mature, will bring new insight to the ASR reaction, allowing for advancements in design, detection and mitigation of ASR, and eventually has the potential to become a method-of-choice for in-situ infrastructure health-monitoring of existing structures --Abstract, page v

Effect of Exposure Conditions on the Long-Term Dielectric Properties of Mortar Samples Containing ASR Gel

Alkali-silica reaction (ASR) is a chemical reaction between alkalis present in portland cement and amorphous or otherwise disordered siliceous minerals in particular aggregates. Through this reaction, reactive silica binds with hydroxyl and alkali ions and forms a gel, known as ASR gel. Recently, microwave materials characterization techniques have shown great potential for detecting ASR in mortar. However, the comprehensive understanding of variables that affect the extent of ASR in mortar and their interaction with microwave signals, in particular the effect of environmental exposure conditions requires more investigations. Therefore, parameters related to these conditions must be considered when using microwave techniques for ASR detection and evaluation. In this paper, the effect of exposure conditions on ASR gel formation and microwave dielectric properties of mortar samples is investigated. To this end, extended measurements of the complex dielectric constants of three different sets of mortar samples are presented at S-band (2.6-3.95 GHz). The samples were cast with potentially reactive ASR-aggregates and subjected to different environmental conditions. The results show slightly different permittivities for the differently stored samples, potentially indicating different amount of ASR gel. This observation was corroborated through UV fluorescence microscopy, where different amounts of ASR gel were observed in the samples. Moreover, the results indicate that ASR gel evolution may be better tracked through loss factor measurements, while pre-existing-gel may be better detected through permittivity measurements

Characterization and Evaluation of Ground Glass Fiber as a Cementitious Component in Portland Cement and Geopolymer Concrete Mixtures

A large amount of glass fiber is commercially produced for use in various applications. However, this process generates millions of tons of waste glass fiber annually around the world. This material has an amorphous structure that is rich in silica, alumina and calcium oxides, and if milled into a fine powder, it could potentially be used a supplementary cementitious material (SCM) in portland cement mixtures; or as a source material for production of geopolymer. So, the first objective of this research work, is to evaluate the utilization of ground glass fiber (GGF) as a SCM in portland cement mixtures, and the second objective is to study the mechanical and durability properties of GGF-based geopolymers. To fulfill the first objective, concrete and mortar mixtures containing different dosage of GGF (i.e. 10, 20 and 30% by mass) were prepared. Fresh and hardened properties of these mixtures were tested and compared with two control mixtures, including: (i) a mixture made from 100% portland cement, and (ii) a mixture having 75% portland cement and 25% class F fly ash (by mass). It was observed that utilization of GGF up to 30% (as a cement replacement) did not influence the mechanical properties of the concrete and mortar mixtures significantly compared to control mixtures; however, the use of GGF as SCM resulted in a remarkable improvement in the durability of the mixtures. It was also seen that the utilization of GGF at the 30% replacement level, successfully mitigated the ASR-related expansion of mortar and concrete mixtures containing the crushed glass aggregate. For the second objective, the possibility of producing geopolymer from GGF was investigated. To activate GGF, different dosage and combinations of sodium hydroxide solution (NaOH) and sodium silicate solution were used, and specimens were cured at 60oC for 24 h. Fresh and hardened properties of geopolymer mixtures made from GGF as the precursor, were studied and compared to glass-powder (GLP) and fly ash-based geopolymer mixtures. The effect of change in the Na2O-to-binder ratio (alkali content of the activator solution) and the SiO2/Na2O (silica content of the solution) ratio on the workability of and compressive strength of the mortar mixtures was monitored and compared to the GLP and fly ash-based geopolymers. It was seen that the strength gain in GGF-based geopolymers does not depend on the presence of sodium silicate in the activator solution; and a high compressive strength (as high as 80 MPa) can be achieved in three days, only by using sodium hydroxide solution alone. Furthermore, to better understand parameters affecting the activation of GGF-based geopolymers, effect of temperature (from ambient to 110oC) and duration of heat-curing on the compressive strength and micro-structure of GGF-based geopolymers was studied. The temperature of heat curing was seen to affect the early-age (i.e. 3 to 7 days) compressive strength of the GGF-based samples but had no significant effect on the later-age (i.e. 28 to 56 days) strength. Finally, it was concluded that GGF has a good potential to be used as a precursor to produce high strength geopolymers even at ambient temperature (23oC). Based on the results obtained from the compressive strength experiments, mixtures with the highest compressive strength were selected from each precursor to be used for the durability experiments. Durability aspects of GGF-based geopolymer such as resistance against sodium sulfate solution and magnesium sulfate solution, alkali silica reaction, drying shrinkage and corrosion of steel rebar were investigated and were compared to fly ash and GLP-based geopolymer, and an ordinary portland cement mixture (OPC). Based on this investigation it was found that GGF and fly ash-based geopolymers showed superior performance against ASR-related deterioration in comparison to GLP-based geopolymer and the OPC mixture. It was also observed that despite the fluctuation in properties at early ages, immersion in the sodium sulfate (Na2SO4) solution and magnesium sulfate (MgSO4) solution did not lead to a significant mass or strength loss of GGF-based geopolymer at the later ages. In conclusion, it can be stated that a high compressive strength GGF-based geopolymers could be produced by using an activator solution that is comprised of only NaOH. Durability experiments conducted on GGF-based geopolymer mixtures showed good performance in resisting ASR and sulfate solution exposure. Based on preliminary results it was observed that drying shrinkage of GGF and fly ash-based geopolymer was similar to the OPC mixture while the drying shrinkage of GLP-based geopolymer was significantly higher. Findings from basic experiments conducted in this study showed that factors such as: (i) the low amount of CH in the structure, (ii) low porosity, and (iii) the durable structure of the geopolymer gel in the GGF-based geopolymers, which remains stable under the aggressive conditions such as, exposure to sulfate solutions, are responsible for the superior durability performance of GGF-based geopolymer

Revisiting Gordion\u27s Pebble Mosaic Pavement: Evaluating Re-Backing Techniques and Investigating Alkali-Silica Reaction

The 9th c. B.C.E. Megaron 2 pebble mosaic from Gordion is the oldest known mosaic pavement to date. Discovered in 1956, it was subsequently cut into 33 individual panels, lifted, and stored outdoors for two decades before reinstallation under a sheltered, sub-grade, outdoor exhibit at the Gordion Museum. Recent research into its conservation history and current condition in preparation for re-interpretation and display has revealed the potential for further deterioration through alkali-silica reaction between the siliceous pebbles and the cementitious backing. Moreover, the enigmatic geometric designs are now difficult to read due to the loss of pebbles, misalignment of the panels, fragmentation by lacunae and gaps between the panels, rebar cracking, and cementitious over-grout. These conditions have created a critical situation that must be remedied to preserve this historically significant mosaic and make it more readily available for interpretation and exhibition. This paper assesses materials and techniques for re-backing the mosaic and investigates the potential for alkali-silica reaction. This includes empirical tests on replica mosaic panels for the removal of the reinforced concrete and cementitious over-grout, the evaluation of critical properties related to the facing and re-backing materials, and petrographic analysis for the detection of evidence of alkali-silica reaction. The analysis, testing, and proposed treatments for this significant archaeological pavement is presented in light of contemporary conservation approaches for ancient tessellated pavements and explores the limits to current knowledge and practice when applied to a natural pebble mosaics

Influence of Moisture on Alkali Silica Reaction

Alkali silica reaction has been discovered over 75 years ago as a deleterious reaction in concrete, but the mechanism is not yet fully understood. Although there have been thousands of studies over the years, most of these studies have focused mostly on accessing aggregates that are susceptible alkali silica reaction so as to avoid it in new structures. In this study, focus is placed on understanding how moisture affects the progression of alkali silica reaction, as moisture cannot be affect the internal relative humidity inside concrete eliminated in concrete compared to other prerequisites for alkali silica reaction in concrete.Understanding how various forms of moisture will affect the reaction will help to understand the reaction more and how to sustain structures already affected by ASR. This study employs the use of relative humidity as a measure of moisture in concrete, four sensors were assessed and internal relative humidity in concrete samples was measured at different depths to see how the external conditions affect the internal moisture of the concrete. Also, the effects of the initial water added to concrete, i.e. the water to cement ratio, and external relative humidity was studied to see how they affect expansion due to ASR. The overall objective of this thesis is to observe how moisture in various ways affects progression of alkali silica reaction in terms of primarily expansions observed and visual inspection of samples under an optical microscope. It was observed that the external relative humidity has an effect on the internal moisture in concrete. Also, it was observed that the initial water to cement ratio has a slight increase on ASR as expansion of samples increase in water to cement ratio. It was also noted that fly ash and coatings affect the internal relative humidity inside concret

Advances in understanding alkali-activated materials

Alkali activation is a highly active and rapidly developing field of activity in the global research and development community. Commercial-scale deployment of alkali-activated cements and concretes is now proceeding rapidly in multiple nations. This paper reviews the key developments in alkali-activated materials since 2011, with a particular focus on advances in characterisation techniques and structural understanding, binder precursors and activation approaches, durability testing and design, processing, and sustainability. The scientific and engineering developments described in this paper have underpinned the on-going scale-up activities. We also identify important needs for future research and development to support the optimal and appropriate utilisation of alkali activated materials as a component of a sustainable future construction materials industry

Mitigating alkali silica reaction in recycled concrete

Concrete made with recycled concrete aggregate (RCA) that had shown alkali silica reaction (ASR) distress was evaluated. It was found that ASR mitigation was needed to prevent continued expansion. Several mitigation methods were evaluated and compared to conventional concrete using the testing procedures of ASTM C 1260/1567 and ASTM C 1293. Pore solution analysis and Thermal Gravimetric Analysis (TGA) were also conducted. It was found that fly ash, silica fume and ground granulated blast furnace slag (GGBFS) were effective in mitigating ASR in RCA concrete. Compared to conventional concrete, slightly higher dosages were typically required. The ASTM C 1260/1567 test correlated well with ASTM C 1293 with few exceptions. Pore solution and TGA revealed that calcium hydroxide and alkalis are reduced by fly ash, silica fume and GGBFS substitution. Calcium depletion during the pozzolanic reaction is a sufficient condition for ASR arrest, but the alkali reducing effect appears to be more pivotal than calcium depletion. Lithium, as well as the other mitigation strategies, required higher dosages with RCA concrete than conventional concrete. The pore solution lithium to alkali ratio was found to be lower and at one year, reaching equilibrium at approximately 50 to 60 percent of the original dose. Modifying the soak solution of ASTM C 1260 resulted in higher levels of lithium in the test samples and made the test less conservative. Topical application of lithium nitrate solution showed reduced surface expansion of pavement blocks however there was no effect on inside expansion. Lithium was found to penetrate about 25mm

The use of electrical impedance spectroscopy for monitoring the hydration products of Portland cement mortars with high percentage of pozzolans

In this paper, mortars and pastes containing large replacement of pozzolan were studied by mechanical strength, thermogravimetric analysis (TGA), scanning electronic microscopy (SEM), mercury intrusion porosimetry (MIP) and electrical impedance spectroscopy (EIS). The effect of metakaolin (35%) and fly ash (60%) was evaluated and compared with an inert mineral addition (andalusite). The portlandite content was measured, finding that the pozzolanic reaction produced cementing systems with all portlandite fixed. The EIS measurements were analyzed by the equivalent electrical circuit (EEC) method. An EEC with three branches in parallel was applied. The dc resistance was related to the degree of hydration and allowed us to characterize plain and blended mortars. A constant phase element (CPE) quantified the electrical properties of the hydration products located in the solid¿solution interface and was useful to distinguish the role of inert and pozzolanic admixtures present in the cement matrix.The authors thank the Universitat Politecnica de Valencia (UPV, Vicerrectorado de Investigacion) for its support (project PAID-05-09 ref 4302) and Debra Westall (UPV) for revising the manuscript.Cruz González, JM.; Fita Fernández, IC.; Soriano Martinez, L.; Paya Bernabeu, JJ.; Borrachero Rosado, MV. (2013). The use of electrical impedance spectroscopy for monitoring the hydration products of Portland cement mortars with high percentage of pozzolans. Cement and Concrete Research. 50:51-61. doi:10.1016/j.cemconres.2013.03.019S51615

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